“Cannot open” when saving combined videos in QuickTime Player

I have several hundred videos that are each divided into ten 2-minute segments, so I am running an AppleScript to combine these with QuickTime Player 10.4. The way it works is by pasting all the parts into one QuickTime movie (in a single window), then the script keystrokes command+W to close it. Before it closes, a "Do you want to save before closing?" dialog appears, and the script fills in my desired title and presses save.

The script is generally working, but occasionally, I get an error message from QuickTime that says "Cannot open" after the save bar has already made it to the end. When this happens, I am unable to save the combined video, even if I try it again with a different destination or by going through the export menu. (When trying to export after this occurs, I get the error "Cannot Open This media may be damaged.")

This person encountered a similar error, and this was the top answer:

Sounds like a permissions problem. If it’s a bug I suspect it will be fixed in a future macOS update. Not much help, I know.

Until then, you have to peek behind the curtain:

1. Unsaved recordings are stored at

   ~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Autosave Information/

2. The file will be called

   Unsaved QuickTime Player Document.qtpxcomposition

   , or similar

3. Show Package Contents

   on that file
4. Inside is

   Screen Recording.mov

   which you can copy and keep

Unfortunately, this didn't work for me. The .qtpxcomposition file's package contents does not contain an .mov file. It only contains the file "index.qtpx".

In ~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Autosave Information/, I have the following files:

• "com.apple.QuickTimePlayerX.plist"
• "Unsaved QuickTime Player Document.qtpxcomposition"

I can drag the .qtpxcomposition file to another folder, but it remains only 14 KB and QuickTime still cannot save or export it.

The videos I am combining are stored on an external hard drive, and I am saving them to a different external hard drive. I made sure in my System Settings that the hard drives are not going to sleep when inactive. So all of this is to say that I'm stumped as to why this error is occurring, and I would greatly appreciate any feedback.